A variable displacement pressure compensated pump provides a constant rate of flow of working pressure fluid from the pump without exceeding a set maximum pressure. When the pressure of the working fluid output from the pump reaches the maximum pressure setting the displacement control automatically reduces the displacement of the pump until fluid output from the pump has a pressure equal to the maximum pressure setting for the pump. When the pressure of the fluid output from the pump decreases the displacement control increases the displacement of the pump until the pressure of the fluid output from the pump attains the maximum pressure setting for the pump.
The variable displacement pump may include a plurality of pistons mounted in longitudinal bores formed in a rotatably mounted pump barrel. Each piston may have a shoe pivotally attached to a head end which projects from the barrel. The shoes may be retained against a thrust plate formed on one surface of a hanger or rocker cam which pivots within the housing about an axis perpendicular to that of the piston bores. A prime mover may be utilized to rotate the barrel such that the pistons reciprocate as the piston shoes slide across the thrust plate.
This reciprocating movement of the pistons causes fluid to be drawn into the piston bores at low pressure and expelled therefrom at high pressure. The angle of the pivoting thrust plate determines the displacement of the pump. If the thrust plate surface extends in a plane perpendicular to the axis of the piston bores the shoes will not reciprocate as the barrel is driven and no displacement of fluid will occur. Conversely, if the hanger or rocker cam is pivoted such that the thrust plate surface extends other than at a right angle with respect to the piston bores the pistons will reciprocate as the shoes slide across the thrust plate surface and displacement of the fluid will occur. Displacement of the pump increases as the thrust plate surface angle increases from the zero displacement position or the position in which the thrust plate surface extends perpendicular to the rotational axis of the pump.
In a typical pressure compensated variable displacement pump a spring acts against the hanger or rocker cam to bias it towards a position of maximum fluid displacement. The maximum displacement of the pump will be set by a stop which may be adjustable to limit the maximum angle the hanger or rocker cam may pivot away from the zero displacement position. Typically a fluid actuated stroking piston engages the pump hanger or rocker cam and acts in opposition to the spring to reduce the displacement of the pump upon receipt of control fluid from a pressure compensator valve.
A pressure compensator valve may simply comprise a spring and a control element. The spring acts on the control element and functions to set the maximum allowable working pressure of the pump. The opposite end of the control element may be connected to a source of control or working pressure fluid. This fluid acts upon the control element in opposition to the spring. The control element may comprise a spool having a land movable longitudinally within a spool bore. In one position of the control element a fluid conduit connected to the stroking piston is connected to tank, in another position of the control element the fluid conduit connected to the stroking piston is connected to a source of working pressure and in a centered position between the one and the other positions the fluid conduit connected to the stroking piston is blocked. The control element constantly modulates between the one and the other position. In the one position of the control element the pump is at the maximum displacement position. However, in the other position of the control element working pressure fluid flows to the fluid conduit connected to the stroking piston to cause the piston to rotate the hanger or rocker cam to a position of reduced fluid displacement against the action of the hanger spring. Working pressure fluid is supplied to this conduit to reduce the displacement of the pump until the pressure of the working fluid is reduced to the maximum allowable pressure set by the spring acting on the control element. Such a basic pressure compensated pump control may be seen in U.S. Pat. No. 4,289,452.
In a typical hydraulic system the working pressure fluid output from the pump may be utilized to power a plurality of devices. Typically, fluid may be supplied to these devices through a conventional four-way valve. In such a system the demand for working pressure fluid may vary considerably. Rarely does the system call for working pressure fluid at the maximum pressure setting of the pump. Consequently, a great deal of energy and heat may be conserved if the pressure of the working fluid output from the pump is modulated to respond to the demands of the system. Such modulation may be achieved by changing the maximum pressure setting of the compensator valve in response to changes in demands of the system. An electromechanical device for changing the maximum pressure setting of a pressure compensator valve may be seen in U.S. Pat. No. 4,715,788. However, an electromechanical system in most instances would not be used on hydraulic systems which do not have a ready supply of electrical power such as some types of drilling equipment, mining equipment or construction machinery. In these types of systems purely mechanical devices may be utilized.
One problem with a purely mechanical system resides in attempting to modulate the setting of the pressure compensator valve as the load requirements of the hydraulic system varies. The requirements of the hydraulic system may be sensed by examining the outputs of the four-way valves which are connected to the hydraulic devices in the system to be driven by the output of the pump. If these outputs are connected to a single line through a combination of shuttle type check valves that line may act as a load sensing line or port. It has been found to be undesirable to directly connect the load sensing port to the pressure compensator valve to vary the output of that valve in response to the system demand. Such a direct connection requires relatively large fluid passages, e.g. on the order of approximately 0.125 inches or more, valves having large poppets and seats and greatly reduces the response of the system because of the size of the components and because of the relatively large amounts of fluid which must be handled. It has been found desirable to provide a control which utilizes a source of pilot fluid in connection with a pressure compensator valve and which isolates the load sensing fluid from the pressure compensator valve. In this way the compensator valve components may be made very small and the valve may have a high response.